An optical measurement device for irradiating a sample, which is an object to be measured, with excitation light and detecting measurement light has been known in the related art. As this kind of technique, for example, Patent Literature 1 discloses an absolute fluorescence quantum efficiency measurement device that obtains the absorptance of a sample from a measured value of a reflectance obtained using a spectroscopic reflectance standard in an integrating sphere and a measured value of a reflectance obtained using the sample when obtaining internal quantum efficiency (emission quantum yield).
For example, Patent Literature 2 discloses a quantum efficiency measurement device that measures excitation light absorbed in a sample in a state in which excitation light having passed through the sample is reflected in an integration space and measures light generated from the sample in a state in which the excitation light having passed through the sample is not reflected in the integration space. The quantum efficiency measurement device disclosed in Patent Literature 2 reduces a measurement error, which is caused by re-excitation (secondary excitation), by performing two-step measurement processing. Non Patent Literatures 1 to 3 disclose devices that irradiate a sample with excitation light having a predetermined beam cross-section so that the predetermined beam cross-section is surrounded by (covered with) the sample, and calculate internal quantum efficiency and external quantum efficiency internal quantum efficiency×absorptance) on the premise that the area of the sample at an irradiation position of the excitation light (hereinafter, simply referred to as “the area of the sample”) is larger than the predetermined beam cross-section (beam diameter).